Tone-controlled speech scrambler

ABSTRACT

A speech-coding or scrambling system which operates by changing a speech signal by means of an inverter signal, also incorporates a tone signal outside of the speech frequency band to inform the receiver that the speech is being coded. Upon the receipt of the tone signal the receiver automatically operates to decode the received signal.

United States Patent inventor Ronald l-l. Chapman Wheaton, Ill.

Appl. No. 817,696

Filed Apr. 21, 1969 Patented Nov. 30, 197] Assignee Motorola, Inc.

Franklin Park, ill.

TONE-CONTROLLED SPEECH SCRAMBLER [56] Relerences Cited UNITED STATES PATENTS 2,400,950 5/1946 Purington 179/ l .5 M 2,401,403 6/1946 Bedford... 179/1.5 M 2,423,546 7/1947 Bedford... 179/1.5 M 3,012,098 12/1961 Riesz 179/1 .5 M

Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-Malcolm F. Hubler Attorney-Mueller, Aichele 8 Claims, 1 Drawing Fig.

35.5! nuo4l7lllgb5 ABSTRACT: A speech coding or scrambling system which "04!!! 3/16, Operates by changing a speech Signal by means of an invcner meld Search 7971 5 E signal, also incorporates a tone signal outside of the speech 0 5 Fs frequency band to inform the receiver that the speech is being coded. Upon the receipt of the tone signal the receiver automatically operates to decode the received signal.

II l3 i4 I6 I? DOUBLE LP AU l0 D D BAL. ADD. FILTER TRANSMITTER MOD. 5 19 2o 1 i 22 PUSH TO INVERTER TONE TALK SW OSC. GEN. l 1 ON OFF 23 2 i l i l I 32 31 26 25 AUDlO L.P. DOUBLE FILTER RECEIVER 29 mvERTER OSC.

TONE

DECODER PATENTEU NUVBOIQYI 3,624,297

n l3 l4 l6 l7 DOUBLE LfP. I

AUDIO D 3 1 ADD. FILTER 'E'TRANSMITTER 5 l9 2o 1 1 22 I PUSH TO INVERTERQ TONE TALK sw osc. GEN. 1

f ON OFF 23 I sw. I I I i 32 3 26 25 l AUDlO L.P. DOUBLE BAL. RECEIVER FuIER MOD 29 INVERTER osc.

2s TONE DECODER INVENTOR RONALD Hv CHAPMAN ATTYS.

TONE-CONTROLLED SPEECH SCRAMBLER BACKGROUND OF THE INVENTION It has been found desirable in certain communications operations to preserve the privacy of the information being transmitted to prevent unauthorized persons from receiving the information. Since much information is transmitted in plain language by radio it has become difficult to preserve the privacy because of the easy availability of receiving equipment. Sometimes messages have been sent in code so that listeners, unless they have access to means for decoding the message. cannot determine its contents. This, however, is difficult to do with voice transmission, particularly since the recipient of the message and, more often, may not have the time. It has therefore been desirable, with regard to speech transmission, to code or scramble the speech signal at the transmitter and decode the received signal in the receiver, so that the actual message transmitted is in a form which cannot be understood by anyone having an ordinary receiver.

Although various types of speech scramblers have been developed, there are problems in their use. Most speech scramblers which are effective are relatively complex and very costly so that their use is limited to special situations. Simple speech scramblers have been developed such as, for example, a scrambler in which the speech signal is mixed with an inverter signal in a double-balanced modulator to produce inversion of the speech frequencies. The scrambled speech signal is easily decoded by mixing the scrambled signal with the same frequency in the double-balanced modulator at the receiver. The signal used for inverting the frequency is outside of the speech frequency band.

This type of scrambler is effective in making the voice unintelligible and it also meets the requirements of simplicity and low cost. However, its simplicity also makes it easy for an unauthorized listener to unscramble the signal. In order to make this more difficult only those portions of the message which are important enough to have theirsecrecy preserved are transmitted as coded messages. The remaining portions of the message are sent in plain language. In order to accomplish this it is necessary to tell the recipient to shift to the coding mode of operation before starting the coded portion of the message.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide an improved speech-coding system.

Another object of this invention is to provide a speech-coding system in which the receiver is automatically shifted to the decoding mode.

In practicing this invention a speech-coding system is provided which includes an inverter oscillator and a doublebalanced modulator. The speech signal is fed into the doublebalanced modulator together with the inverter oscillator signal. The frequency of the inverter oscillator signal is outside the band of frequencies of the speech signal and acts to invert the frequencies of the speech signal in a manner so that it becomes unintelligible. The output of the double balanced modulator is filtered in a low pass filter and transmitted. The received signal is demodulated and the resulting encoded speech signal is coupled to a double-balanced modulator similar to the transmitter double-balanced modulator. An inverter oscillator developing the same frequency as the inverter oscillator of the transmitter provides a signal which is mixed with the encoded speech signal in the double-balanced modulator to decode the encoded speech signal. The decoded signal is filtered in a low-pass filter and coupled to audio circuits for reproduction thereby.

The transmitter is also provided with a tone generator which can be turned on and off as desired. With the tone generator on, a tone signal, having a frequency outside of the speech band. is generated and is coupled to the transmitter for transmission. When the tone signal is developed the signal from the inverter oscillator is applied to the double balanced modulator to encoded speech signal. Thus, when the transmitted speech signal is encoded a tone indicating that it is encoded is also transmitted.

At the receiver the tone signal is separated from the encoded speech signal and is detected to provide a control signal. The inverter oscillator of the receiver is responsive to this control signal to develop an inverter oscillator signal which is applied to the double-balanced modulator of the receiver. This results in an automatic decoding of the encoded speech signal. Thus, when the operator desires to encode -a portion of his communication he need merely to push a button or operate a switch and the speech is automatically encoded and decoded. No warning need be given the recipient and thus no warning is given to unauthorized persons who are listening. Further, since the duration of the scrambled signal may be short, unauthorized listeners would not have time to search out the correct inverter frequency to try to unscramble the signal.

The invention is illustrated in the single drawing, a block diagram illustrating the operation of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawing, speech signals are developed by -a microphone l0 and are amplified and processed in the audio circuits 11. The speech signal output of audio circuits I1 is coupled to a double-balanced modulator 13. The output of the double-balanced modulator 13 is coupled through an adder I4 and a low pass filter 16 to a transmitter 17. The speech signal modulates a carrier in transmitter 17 and the resultant carrier signal is transmitted from transmitter 17 to the receiver 25. The modulated carrier signal is processed in receiver 25 to separate the modulating signal from the carrier signal. The output of receiver 25 is coupled to a double-balanced modulator 26 similar to the double-balanced modulator 13. The output of the double-balanced modulator 26 is coupled through a low-pass filter 31 to audio circuitry 32 for reproduction thereby.

The above describes the operation of the circuit without encoding. To encode the speech signal to provide privacy, an inverter oscillator 20 and a tone generator 22 are provided. The inverter oscillator and the tone generator both provided signals outside the range of the speech signal, and the frequency of the tone generator can differ from the frequency of the oscillator inverter by the order of cycles.

When it is desired to transmit an encoded speech signal, the on-off switch 23 is placed in the on position to apply an activating signal to the tone generator 22. Push-to-talk switch 19, which is operated when it is desired to transmit speech, activates transmitter 17. In addition, with push-to-talk switch 19 operated another activating signal is applied to tone generator 22. Thus every time the push-to-talk switch 19 is operated, an activating signal is sent to tone generator 22. However, the tone generator 22 is not activated unless the on-ofi switch 23 is in the on position. The tone generator 22 activates the inverter oscillator 20.

With the push-to-talk switch 19 operated and on-off switch 23 in the on position, inverter oscillator 20 responds to the tone signal and develops a coding signal which is coupled to the double-balanced modulator 13. The frequency of the coding signal is outside of the frequency band of the speech signals so that it does not become part of the speech signal. The double balanced modulator l3 acts to develop sum and difierence signals which are coupled to adder 14. The control tone output of tone generator 22 is added to the sum and difference signals in adder l4 and the resulting signals are coupled through low-pass filter I6 to transmitter 17. Low-pass filter l6 acts to remove the sum signals so that only the difference signals and the tone from tone generator 22 are transmitted. The difference signals act to invert the frequencies of the speech signal depending upon the frequency of the inverter oscillator 20.

The transmitted signal is received by receiver 25 and the encoded speech signal and tone signals are separated from the carrier signal. The encoded speech signal and tone signals are coupled to the double-balanced modulator 26 and tone decoder 28. Tone decoder 28 detects the presence of the control tone signal and develops a control signal which activates inverter oscillator 29. Inverter oscillator 29 develops a signal having the same frequency as inverter oscillator and the output signal from inverter oscillator 29 is coupled to double balanced modulator 26. The encoded speech signal and the signal from inverter oscillator 29 are mixed in doublebalanced modulator 26 to develop sum and difierence frequencies. The sum frequencies are removed by low-pass filter 31 to develop the decoded speech signal. The decoded speech signal is coupled to audio circuits 32 for reproduction thereby.

The use of a separate tone to activate the unscrambling circuit of the receiver acts to increase the flexibility of the frequency inversion coding technique. For example, there may be many receivers which have the decoding circuitry and it may be desirable to transmit to only one or a small group of these receivers at one particular time. In prior systems it would be necessary to have a separate inverter oscillator frequency for each receiver or group of receivers. It has been found that in order to achieve privacy it is necessary to space the inverter oscillator signals a minimum of 300 cycles per second apart. The tones from tone generator 22, indicating that the speech is being coded, can be spaced 100 cycles per second apart. Thus each receiver could have the same inverter frequency signal but only those desired would be activated by the tone signals from tone generator 22.

The operator can control the coding by the operation of a switch to provide intennittent coding, to thereby increase the privacy. The receiver automatically follows the coding, and the reception of the speech is not affected by the coding action. An efi'ective privacy system is therefore provided by quite simple equipment.

What is claimed is:

l. A speech-coding system including in combination, tone signal generator means for producing a tone signal, switch means coupled to said tone signal generator means for causing selective operation thereof to develop the tone signal, speechcoding means adapted to receive a speech signal and coupled to said tone signal generator means, said speech-coding means being responsive to said tone signal to encode said speech signal, said speech-coding means further acting to combine said encoded speech signal and said tone signal, and transmitting means coupled to said speech-coding means for transmitting said combined signal.

2. The speech-coding system of claim 1 wherein said speech-coding means includes a double-balanced modulator to which the speech signal is applied, and oscillator means coupled to said modulator and to said tone signal generator means, said oscillator means being responsive to said tone signal to apply oscillations to said modulator.

3. The speechcoding system of claim 2 wherein said speech-coding means includes adder means coupled to said double-balanced modulator means to combine the output thereof with said tone signal, and low-pass filter means coupling said adder means to said transmitting means.

4. A speech-coding system including in combination, tone signal generator means selectively operated to develop a tone signal, speech-coding means coupled to said tone signal generator means and adapted to receive an applied speech signal, said speech-coding means being responsive to said tone signal to encode said speech signal, said speech-coding means further acting to combine said encoded speech signal and said tone signal, transmitting means coupled to said speech-coding means for transmitting said combined signal, receiver means for receiving said combined signal and for reproducing said encoded speech signal and said tone signal, tone signal decoder means coupled to said receiver means and responsive to said tone signal to develop a control signal, and speechdecoding means coupled to said receiver means and to said tone signal decoder means and responsive to said control signal to decode said encoded speech signal to develop the applied speech signal.

5. The speech-coding system of claim 4 wherein, said speech-coding means includes a first double-balanced modulator for receiving the applied speech signal, and first oscillator means coupled to said first double-balanced modulator and to said tone signal generator means, said first oscillator means being responsive to said tone signal to develop a first oscillator signal, said first double-balanced modulator acting to mix said speech signal and said first oscillator signal.

6. The speech-coding system of claim 5 wherein, said speech-coding means includes adder means coupled to said first modulator and to said tone signal generator means and acting to combine said mixed signal and said tone signal, and first low-pass filter means coupling said adder means to said transmitting means for coupling only those signals below a predetermined frequency from said adder means to said transmitting means.

7. The speech-coding system of claim 6 wherein said speech-decoding means includes second double-blanaced modulator means coupled to said receiver means, second oscillator means coupled to said second double-balanced modulator means and said tone signal decoder means, and second low-pass tilter means coupled to said double-balanced modulator means, said second oscillator means being responsive to said control signal to develop a second oscillator signal, said second double-balanced modulator acting to mix said encoded speech signal and said second oscillator signal, said second low-pass filter means acting to filter said mixed second oscillator signal and said encoded speech signal to develop the applied speech signal.

8. The speech-coding system of claim 4 wherein said speech decoding means includes oscillator means coupled to said tone signal decoder and responsive to said control signal to develop an oscillator signal and modulator means responsive to said oscillator signal to decode the received encoded speech signal. 

